Superior collicular projection to intralaminar thalamus in rat

The superior collicular (SC) cells which project to the intralaminar thalamus (IT; nuclei centralis lateralis, CL; paracentralis, PC; parafascicularis, Pf) in the rat were identified by means of retrograde transport of wheatgerm agglutinin conjugated horseradish peroxidase (WGA-HRP). SC-IT cells were located throughout the mediolateral and rostrocaudal extents of the tectum ipsilateral to the thalamic injection. In this SC, they had a primarily bilaminar distribution in the lower one-half of the stratum griseum intermediale (SGI) and upper portion of the stratum griseum profundum (SGP). In these laminae, SC-IT cells were arranged in clusters or patches similar to those which have been described for many inputs to the deep SC laminae. A small number of SC-IT cells were also observed in the deep laminae of the tectum contralateral to the thalamic injection. Double labelling experiments using True Blue (TB) and Diamidino Yellow (DY) demonstrated that less than 1% of the contralaterally projecting SC-IT cells also innervated ipsilateral IT. Anterograde tracing with [3H]leucine demonstrated further that SC projected heavily to CL, PC and Pf. This projection also extended into the medial portion of the posterior thalamus (PO).